Systems and methods for real-time inductor current simulation for a switching converter

ABSTRACT

A switching converter having a high-side switching transistor and a low-side switching transistor and an inductor, having a circuit for generating a simulated waveform representing a sawtooth inductor current waveform. A circuit for monitoring and voltage at a switch node between the high-side and low-side transistors to determine a time during which the inductor current is increasing and a time during which the inductor current is decreasing wherein voltage across the low-side transistor when it is conducting represents a first portion of the simulated sawtooth inductor current waveform. A circuit for utilizing the time when the inductor current is increasing, the time when the inductor current is decreasing and the voltage across the low-side transistor when it is conducting to generate a portion of the simulated inductor current waveform when the high-side transistor is conducting. A method and a power supply utilizing this circuit are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Nonprovisional patentapplication Ser. No. 13/327,146, filed Dec. 15, 2011 (which will issueas U.S. Pat. No. 8,866,464), and claims priority from U.S. ProvisionalApplication No. 61/950,443, filed Mar. 10, 2014 both of which are herebyincorporated by reference in their entirety for all purposes.

TECHNICAL FIELD

The present invention relates generally to electronics, and specificallyto systems and methods for regulating a switching converter.

BACKGROUND

Switching regulators have been implemented as an efficient mechanism forproviding a regulated output in power supplies. One such type ofregulator is known as a switching supply circuit, which controls theflow of power to a load by controlling the “ON” and “OFF” duty-cycle ofone or more high-side switches coupled to the load.

Many different classes of switching supplies exist today. One type ofswitching supply circuit is known as a synchronous switching supplycircuit. In a synchronous switching supply circuit, an inductor is usedto maintain current flow that is switched from two separate sources. Thetwo sources can include a high-side switch, such as a high-sidefield-effect transistor (FET), and a low-side switch, such as a low-sideFET. After the high-side FET is deactivated, the low-side FET becomesactivated. The low-side FET thus conducts current from ground to theinductor because magnetic power stored in the inductor dissipates toforce current through the inductor by changing the voltage of theinductor source node to negative relative to ground. In this way,current continuously flows through the inductor, even at times when thehigh-side switch is deactivated.

It is desirable in the design of switching supplies to ensure that theoutput of the switching supply circuit is properly regulated. Forexample, if a load at the output of the switching supply circuitchanges, it may be necessary to change the switching operation, such asby adjusting the switching duty-cycle, to regulate the output voltage toa relatively constant level. Regulation is typically accomplishedthrough feedback control, by either a voltage feedback technique, inwhich the output voltage of the switching supply is monitored, or acurrent feedback technique, in which both the output voltage and theinductor current are monitored. The current feedback technique canmonitor the inductor current by connecting a current sense resistor inseries with the output inductor. However, a resistor connected in serieswith the output inductor can result in a degradation of the performanceefficiency of the switching supply circuit.

Another way to accomplish the current feedback technique is by employinginductor direct current resistance (DCR) sensing, which is determiningthe inductor current by measuring the voltage drop across the parasiticresistance of the inductor. However, DCR sensing has several pitfalls,such as requiring external temperature compensation, dealing with largeDCR tolerances that limit overall accuracy, the need to have additionalrouting from the integrated circuit to power components and theinability to utilize minimum DCR inductors to preserve signal integrity.

SUMMARY

In an aspect in a switching converter having a high-side switchingtransistor and a low-side switching transistor and an inductor, acircuit for generating a simulated waveform representing a sawtoothinductor current waveform. A circuit for monitoring voltage at a switchnode between the high-side and low-side transistors determines a timeduring which the inductor current is increasing and a time during whichthe inductor current is decreasing wherein voltage across the low-sidetransistor when it is conducting represents a first portion of thesimulated sawtooth inductor current waveform. A circuit for utilizingthe time when the inductor current is increasing, the time when theinductor current is decreasing and the voltage across the low-sidetransistor when it is conducting to generate a portion of the simulatedinductor current waveform when the high-side transistor is conducting.

Another aspect includes a method of operating a switching converterhaving a high-side switching transistor and a low-side switchingtransistor and an inductor and having a circuit for generating asimulated waveform representing a sawtooth inductor current waveform.Monitoring a voltage at a switch node between the high-side and low-sidetransistors to determine a time during which the inductor current isincreasing and a time during which the inductor current is decreasingwherein voltage across the low-side transistor when it is conductingrepresents a first portion of the simulated sawtooth inductor currentwaveform. Utilizing the time when the simulated inductor currentwaveform is increasing, the time when the simulated inductor currentwaveform is decreasing and the voltage across the low-side transistorwhen is conducting to generate a portion of the simulated inductorcurrent waveform when the high-side transistor is conducting.

A further aspect includes a power supply including a switching converterhaving a high-side switching transistor, a low-side switchingtransistor, an inductor and a control circuit for implementing currentcontrol of the power supply. A circuit for monitoring a voltage at aswitch node between the high-side and low-side transistors to determinea time during which the inductor current is increasing and a time duringthe inductor current is decreasing wherein the voltage across thelow-side transistor when it is conducting represents a first portion ofa simulated sawtooth inductor current waveform. The control circuitutilizing the time when the inductor current is increasing, the timewhen the inductor current is decreasing and the voltage across thelow-side transistor when it is conducting to generate a portion of thesimulated inductor current waveform when the high-side transistor isconducting, the control circuit implementing current control of thepower supply output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a power supply system that includes a switchingsupply circuit and a simulated output generator in accordance with anaspect of the present invention.

FIG. 2 illustrates an example of a simulated output generator inaccordance with an aspect of the present invention.

FIG. 3 illustrates exemplary waveforms of the system of FIG. 1 andsimulated output generator of FIG. 2 in accordance with an aspect of thepresent invention.

FIG. 4 illustrates a method of regulating a switching converter inaccordance with an aspect of the present invention.

FIG. 5 illustrates a technique for compensating for the temperaturecoefficient of Rdson constructed according to the principles of thepresent disclosure.

DETAILED DESCRIPTION

Systems and methods are provided for regulating a power supply. Thesystems and methods provide a simulated inductor waveform as a feedbacksignal to the controller of a switching converter based on a low-sideoutput waveform of a low-side transistor measured at a switching nodeduring off-times of the switching converter. An amplified, invertedversion of the low-side output waveform can be employed to determine ahigh-side portion of the simulated inductor waveform during on-times ofthe switching converter. The simulated inductor waveform is a real timesignal representation of an actual output inductor current andeliminates the need for inductor direct current resistance (DCR)sensing. The systems and methods also eliminate the need for externaltemperature compensation, allow for the use of low DCR inductors thatimprove overall efficiency, and allow for more accurate cycle-by-cycleregulation and improved load transient responses.

FIG. 1 illustrates a power supply system 10 that includes a switchingsupply circuit 12 and a simulated output generator 16 in accordance withan aspect of the present invention. The switching supply circuit 12could be, for example, a synchronous buck or a synchronous boostconverter, and could be a self-oscillating or a fixed-frequencypulse-width modulation regulator. The switching supply circuit 12includes a driver control circuit 14 that controls the operation of ahigh-side field-effect transistor (FET) Q1 and a low-side FET Q2. Thehigh-side FET Q1 and the low-side FET Q2 are demonstrated in the exampleof FIG. 1 as N-type FETs. However, other types of transistors could beused in accordance with an aspect of the invention. The high-side FET Q1is interconnected between a positive voltage rail VI at a drain terminaland a switching node VSW at a source terminal. The low-side FET Q2 isinterconnected between the switching node VSW at a drain terminal and anegative voltage rail at a source terminal, the negative voltage railbeing demonstrated as ground in the example of FIG. 1. The drivercontrol circuit 14 thus controls the voltage potential at the switchingnode VSW by alternately switching between opposing “ON” and “OFF” statesof the high-side FET Q1 and the low-side FET Q2. It is to be understoodthat the opposing switching of the high-side FET Q1 and of the low sideFET Q2 is such that only one of the high-side FET Q1 and the low sideFET Q2 may be activated at a given time to avoid a short circuit betweenthe positive rail VI and ground.

The switching supply circuit 12 also includes an output inductor L1coupled to the switching node VSW through a load resistor RL. The outputinductor L1 is interconnected between the switching node VSW and theoutput VOUT of the switching supply circuit 12, the output VOUT beingcoupled to ground by a series connected resistor RC and capacitor C bothcoupled in parallel with resistor R. The output inductor L1 maintainscurrent IL flowing to the output of the switching supply circuit 12, asdescribed above. Because the load at the output of the switching supplycircuit 12 may change in response to the operation of other circuitcomponents to which the switching supply circuit 12 is supplying power,the output of the switching supply circuit 12 needs to be regulated tomaintain relatively constant output voltage VOUT. Accordingly, theswitching supply circuit 12 includes a simulated output generator 16that supplies feedback to the driver control circuit 14, the feedbackbeing a simulated output signal that is proportional to the outputcurrent signal IL.

The simulated output generator 16 receives inputs from the switchingnode VSW during activation of the low-side FET Q2 and deactivation ofthe high-side FET Q1 (referred to as off-times), and measures a low-sideoutput waveform of the low-side FET Q2, which could be a voltagewaveform (VdsonQ2) across the low-side FET. The simulated outputgenerator 16 then generates a simulated inductor current waveform(ILGEN) that is proportional to the inductor output current IL byemploying an amplified, inverted version of the low-side output waveformand determining a high-side portion (referred to as the “on-time”) ofthe simulated inductor current waveform utilizing the slope and minimumvalues of the amplified, inverted low-side output waveform.Alternatively, the simulated output signal could be a voltage waveformthat corresponds approximately to the output current IL. It is to beappreciated that the simulated output generator 16 can amplify andtemperature compensate the low-side output waveform. The driver controlcircuit 14 receives the simulated output signal as feedback for thepurpose of regulating the output voltage VOUT, for example, bycontrolling the amount of time (e.g., duty-cycle) that the high-side FETQ1 is “ON” relative to the low-side FET Q2.

FIG. 2 illustrates an example of a simulated output generator 40 inaccordance with an aspect of the present invention. The simulated outputgenerator 40 includes a temperature compensated gain and invertercircuit 42 that receives inputs from the switching node VSW duringactivation of the low-side FET Q2 and deactivation of the high-side FETQ1 (i.e., during off-times), and measures a low-side output waveform ofthe low-side FET Q2, which is a voltage waveform (VdsonQ2) across thelow-side FET Q2, as illustrated in waveform 70 of FIG. 3. Thetemperature compensated gain and inverter circuit 42 can include currentsources and other components that are tuned to compensate fortemperature variation during operation. The temperature compensated gainand inverter circuit 42 also provides gain to the low-side outputwaveform since this signal is a low voltage level signal (e.g., +/−50mV). The temperature compensated gain and inverter circuit 42 invertsthe low-side output waveform and sets the gain of the signal tosubstantially match the slope of the actual output current IL, asillustrated in waveform 72 of FIG. 3, through the inductor L1 during theoff-time of the switching converter 12.

The temperature compensated, amplified and inverted low-side outputwaveform is provided to a buffer 43 formed of a first level shifter 44and a second level shifter 46. The first level shifter 44 provides for alow impedance translation and is configured to shift the signal suchthat the signal has a negative voltage rail set to a user defined valuegreater than 0V, so as to provide for the capturing of negative currentinformation. The output of the second level shifter 46 provides a lowimpedance amplified, inverted version of the low-side output waveformreferred to an internally reproduced low-side FET voltage waveform(VdsonGEN) (i.e., IL*Rdson(Q2)), as illustrated in the waveform 74 ofFIG. 3. The output of the second level shifter 46 also provides asimulated inductor current waveform (ILGEN), as illustrated in waveform76 of FIG. 3 that is proportional to the actual inductor output currentwaveform IL, as illustrated in waveform 72 of FIG. 3. The simulatedinductor current waveform (ILGEN) is generated with a waveform generator50 and a charging capacitor C1.

The waveform generator 50 includes a first control loop 52 thatregulates the slopes of the simulated inductor current waveform (ILGEN)with the slopes of the internally reproduced low-side FET voltagewaveform (VdsonGEN), such that the first control loop 52 controls themodulation of the charging of the capacitor C1 during on-times. Thewaveform generator 50 includes a second control loop 54 that regulatesthe minimum values of the simulated inductor current waveform (ILGEN)with the minimum values of the internally reproduced low-side FETvoltage waveform (VdsonGEN), such that the second control loop 54controls the modulation of the discharging of the capacitor C1 duringoff-times. The first control loop forces the simulated inductor currentwaveform (ILGEN) to intersect the actual inductor output currentwaveform IL in the middle of the defined regulation period, while thesecond control loops pulls in the simulated inductor current waveform(ILGEN) to match the actual inductor output current waveform IL. Thefirst and second control loops 52 and 54 in conjunction with thecharging and discharging of capacitor C1 facilitate the generation ofthe simulated inductor current waveform (ILGEN) that is proportional tothe actual inductor output current waveform IL. The switch S1 is coupledbetween the second control loop 54 and the charging capacitor C1 and iscontrolled by a control signal CTRL1, which is derived from a switchingsignal that controls the switching of the high-side FET and/or thelow-side FET.

The first control loop 52 includes a first transconductance amplifier(A1) that receives as inputs the simulated inductor current waveform(ILGEN) and the internally reproduced low-side FET voltage waveform(VdsonGEN) directly from the second level shifter 46. The output of thefirst control loop 52 is coupled to the charging capacitor C1, which iscoupled between the first level shifter 44 and the second level shifter46, and modulates the charging of the charging capacitor C1. The outputof the first transconductance amplifier (Al) is coupled to a transistorQ3, resistor R1 and compensation network capacitor C3 arrangement whichis in turn coupled to an arrangement of current sources IC1-IC3, whichall cooperate to perform the functions of the first control loop 52. Forexample, as illustrated in FIG. 3, the first control loop 52 functionsto regulate the slopes of T0, T1, T2 of simulated inductor currentwaveform (ILGEN) 76 with the respective slopes S0, S1 and S2 of theinternally reproduced low-side FET voltage waveform (VdsonGEN) 74.

The second control loop 54 includes a second transconductance amplifier(A2) that receives as inputs the sampled minimum values of the simulatedinductor current waveform (ILGEN) and the sampled minimum values of theinternally reproduced low-side FET voltage waveform (VdsonGEN). Thesecond control loop 54 receives inputs from a sample-and-hold device 48that includes a first sample-and-hold circuit that samples the simulatedinductor current waveform (ILGEN) output from the second level shifter46 and a second sample-and-hold circuit that samples the internallyreproduced low-side FET voltage waveform (VdsonGEN) from the secondlevel shifter 46. The sample-and-hold device 48 is controlled by acontrol signal CTRL2, which is derived from a switching signal thatcontrols the switching of the high-side FET and/or the low-side FET. Inthis manner, the first sample-and-hold circuit attempts to sample thesimulated inductor current waveform (ILGEN) output at its minimum valuesand the second sample-and-hold circuit attempts to sample the internallyreproduced low-side FET voltage waveform (VdsonGEN) output at itsminimum values.

The output of the second control loop 54 is coupled to the chargingcapacitor C1 through the switch S1 and modulates the discharging of thecharging capacitor C1. The output of the second output transconductanceamplifier (A2) is coupled to a transistor Q4, resistor R2 andcompensation network capacitor C4 arrangement which is in turn coupledto an arrangement of current sources IC4-IC7, which all cooperate toperform the functions of the second control loop 54. For example, asillustrated in FIG. 3, the first control loop 52 functions to regulatethe minimum values of M0, M1, M2 of simulated inductor current waveform(ILGEN) 76 with the respective minimum values L0, L1 and L2 of theinternally reproduced low-side FET voltage waveform (VdsonGEN) 74.

Operation of the circuit having been shown above, below is anexplanation of how and why this control scheme performs the desiredcontrol. The timing signals known through the VSW node in effect splitthe synthesized inductor current to two triangles 80, 82, as shown inFIG. 3 in waveform 72. The first triangle 80 represents the time whentransistor Q1 (FIG. 1) is on and the second triangle 82 represents atime when transistor Q2 (FIG. 1) is on. The base of triangle 80 is 84and a hypotenuse of triangle 80 is 86. The base of triangle 82 is 90 anda hypotenuse of triangle 82 is 88. Referring to triangle 82, thehypotenuse of the triangle is known from the internally reproducedsignal Vdson_(GEN) which is measured by multiplying the inductor currentI_(L) by the resistance of the transistor Rdson, as shown in FIG. 3.Since the base of the triangle 90 is known from the timing signal at theVSW node and since the hypotenuse is known as described above, theheight of the triangle 92 can be determined. Knowing the height andbases of the two triangles, the missing information, which is thehypotenuse 86 of triangle 80 can be determined.

The information from Vdson_(GEN) shown in FIG. 3 as reference 74, isthen translated to IL_(GEN) through the circuit shown in FIG. 2 whichcontains the slope and Valley control loops by forcing the followingconditions, the slope control unit will adjust until T0=S0, T1=S1, andT2=S2, as shown in FIG. 3 by reference numerals 74 and 76; and thevalley control loop will adjust until M0=L0, M1=L1 and M2=L2 as shown inFIG. 3 as reference numerals 74 and 76.

One problem that can occur with this control method is that the errorcan accumulate on a cycle-by-cycle basis. Accordingly, the presentinvention incorporates a cycle-by-cycle DC reset to null out accumulatederror in order to prevent signal “walk-away”. At the initial power-on orenabling of the controller, the produced slopes do not yet matched thoseof the inductor current, which means that the slopes T0≠S0, T1≠S1 andT2≠S2, which also means that the valley signals M0≠L0, M1≠L1 and M2≠L2,see FIG. 3 at reference numeral 74, 76. This in turn can produce anaccumulated error which can prevent the control loops from functioningproperly.

In order to enable proper operation, after the control loops sample thevalley points M0, L0, M1, L1, M2 and L2, a DC reset is performed forcingIL_(GEN) and Vdson_(GEN) to be equal, thus resetting or zeroing theaccumulated error for given cycle. This cycle-by-cycle DC reset preventsIL_(GEN) from walking away to the most positive or most negative voltageallowed by the system (VDD or VSS), and providing incorrect information.This is shown by element 508 in FIG. 5, which is discussed below.Circuits for performing this function are well known to those skilled inthe art and need not be described in detail here.

In view of the foregoing structural and functional features describedabove, certain methods will be better appreciated with reference to FIG.4. It is to be understood and appreciated that the illustrated actions,in other embodiments, may occur in different orders and/or concurrentlywith other actions. Moreover, not all illustrated features may berequired to implement a method.

FIG. 4 illustrates an example of a method 100 for regulating a switchingconverter that includes a high-side transistor and a low-side transistorboth coupled to an output inductor through a switching node. The methodbegins at 102 where the high-side transistor and the low side-transistorare alternately turned on and off with the high-side transistor being onduring on-times and the low-side transistor being on during off-times.At 104, the low-side output waveform of the low-side transistor measuredduring off-times is inverted and amplified. At 106, a high-side portionof a simulated inductor waveform during on-times is determined based onthe amplified, inverted version of the low-side output waveform. At 108,a simulated inductor waveform is generated that is proportional to anactual inductor output waveform through the output inductor based on theamplified, inverted version of the low-side output waveform and thedetermined high-side portion. The methodology then proceeds to 110. At110, slopes of the simulated inductor waveform are regulated with theamplified, inverted version of the low-side output waveform. At 112,minimum values of the simulated inductor waveform are regulated with theamplified, inverted version of the low-side output waveform.

Another problem which can occur with circuit constructed according tothe principles of the present disclosure is that it is dependent uponthe proper value for Rdson. As is well known, the Rdson of a MOSFETtransistor has a positive temperature coefficient. Therefore, it isimportant to temperature-compensate the value utilized in order to getan accurate simulated inductor current measurement waveform. If theswitching transistors are on the same integrated circuit (IC) as thecontrol circuit, a temperature compensation technique utilizing alook-up table which has the variation of Rdson with IC temperature canbe utilized. FIG. 5 shows a circuit for implementing this technique inaccordance with the present disclosure, generally as 500. In FIG. 5, theRdson of the low-side FET 502 is measured by differential amplifier 504,the output of which is fed into the ratio-metric temperaturecompensation circuit 506. Circuit 506 receives a temperature input ofthe temperature of the IC and utilizes it to refer to a look-up table507. The output of ratio -metric temperature compensation circuit 506 isfed to a buffer and cycle-by-cycle DC reset circuit for error 508. Theoutput of buffer 508 is coupled to a sample-and-hold circuit 510 whichstores the value of the voltage measurement and sample-and-hold circuit512 which holds the value related to the output current. The output ofsample-and-hold circuits 510 and 512 are coupled to the inputs of thevalley control loop comprising error amplifier 516. The measured voltageVMEAS and output current IOUT are also coupled to the inputs of theslope control unit comprising error amplifier 514. The output of erroramplifier 516 controls a variable current source 522 and the output oftheir amplifier 514 controls a variable current source 518. Variablecurrent sources 518 and 522 are in series with a VSW controlled switch520. The node at the valley control loop is coupled to an oscillationcapacitor 524.

In operation, the voltage across Q2 created by the current IL and itsinternal resistance Rdson is amplified by differential amplifier 504,the output of which is negative with respect to a reference voltageVRef. The differential output of amplifier 504 is coupled to aratio-metric temperature compensation circuit which receives input ofthe temperature of the IC and utilizes it to obtain a value from thelook-up table 507 as to the variation of Rdson at the measuredtemperature. This is utilized to increase or decrease the gain ofcircuit 506 based upon the temperatures, bearing in mind that Rdson hasa positive temperature coefficient. Therefore, the output ofratio-metric temperature compensation circuit 506, VMEAS, will not varywith respect to temperature. This voltage is then applied to buffer andcycle-by-cycle DC reset for error nulling circuit 508. Thecycle-by-cycle DC reset has been described above. The output of circuit508 is coupled to sample-and-hold circuit 510 and 512. The voltage VMEASis coupled to sample-and-hold circuit 510 and a voltage IOUTrepresentative of the inductor current is coupled to sample-and-holdcircuit 512. As can be seen by the illustrated waveform, the waveformVMEAS shows a portion of the simulated current signal where the waveformIOUT shows the entire waveform. The slope control loop 514 guaranteesthat the slopes of the signal Vdson_(GEN) and IL_(GEN) match. The valleycontrol loop 516 guarantees that the amplitudes of the signals M0=L0,M1=L1, and M2=L2. The slope control loop 514 controls variable currentsource 518 and the valley control loop 516 controls variable currentsource 522. Switch 520 is open when the low side transistor Q2 is on andclosed when the transistor is off. Thus, when this switch is closed,capacitor CT 524 is charged by the difference between current source 518and 522 and, when the switch is open, capacitor CT is discharged by thecurrent source 522. Thus, the waveform IOUT will be generated acrosscapacitor CT 524.

What have been described above are examples of the invention. It is, ofcourse, not possible to describe every conceivable combination ofcomponents or method for purposes of describing the invention, but oneof ordinary skill in the art will recognize that many furthercombinations and permutations of the invention are possible.

Accordingly, the invention is intended to embrace all such alterations,modifications, and variations that fall within the scope of thisapplication, including the appended claims.

What is claimed is:
 1. In a switching converter having a high-sideswitching transistor and a low-side switching transistor and aninductor, a circuit for generating a simulated waveform representing asawtooth inductor current waveform comprising: a circuit for monitoringa voltage at a switch node between the high-side and low-sidetransistors to determine a time during which the inductor current isincreasing and a time during which the inductor current is decreasingwherein voltage across the low-side transistor when is conductingrepresents a first portion of the simulated sawtooth inductor currentwaveform; a circuit for utilizing the time when the inductor current isincreasing, the time when the inductor current is decreasing and thevoltage across the low-side transistor when it is conducting to generatea portion of the simulated inductor current waveform when the high-sidetransistor is conducting.
 2. The circuit for generating a simulatedsawtooth inductor current of claim 1 wherein the time when the inductorcurrent is increasing forms a base of a first triangle of the simulatedinductor current waveform; the time when the inductor current isdecreasing forms the base of a second triangle of the simulated inductorcurrent waveform; and the voltage across the low-side transistor when itis conducting forms the hypotenuse of the second triangle.
 3. Thecircuit for generating a simulated sawtooth inductor current of claim 2wherein a hypotenuse of the first triangle which represents thesimulated inductor current waveform when it is increasing can bedetermined from the values of the bases of the first and secondtriangles and the hypotenuse of the second triangle, whereby an accuratecomplete simulated inductor current waveform can be generated.
 4. Thecircuit for generating a simulated sawtooth inductor current of claim 1further comprising generating a complete simulated sawtooth inductorcurrent waveform from the portion of the inductor current waveform andthe voltage across the low-side transistor when it is conducting;performing a cycle-by-cycle DC reset of the complete simulated waveformand the voltage across the low-side transistor when it is conducting toavoid accumulating an error.
 5. The circuit for generating a simulatedsawtooth inductor current of claim 3 further comprising performing acycle-by-cycle DC reset of the complete simulated waveform and thevoltage across the low-side transistor when it is conducting to avoidaccumulating an error.
 6. The circuit for generating a simulatedsawtooth inductor current of claim 1 wherein the high-side and low-sidetransistors and the circuit for utilizing are on a single integratedcircuit (IC) and comprising: a circuit measuring a temperature of theIC; a look-up table storing values of the low-side transistor Rdson as afunction of the IC temperature; a circuit utilizing the look-up tableand the measured temperature for adjusting the value of Rdson used togenerate the simulated waveform of the voltage across the low-sidetransistor when it is on.
 7. A method of operating a switching converterhaving a high-side switching transistor and a low-side switchingtransistor and an inductor and having a circuit for generating asimulated waveform representing a sawtooth inductor current waveformcomprising: monitoring a voltage at a switch node between the high-sideand low-side transistors to determine a time during which the inductorcurrent is increasing and a time during which the inductor current isdecreasing wherein voltage across the low-side transistor when it isconducting represents a first portion of the simulated sawtooth inductorcurrent waveform; utilizing the time when the simulated inductor currentwaveform is increasing, the time when the simulated inductor currentwaveform is decreasing and the voltage across the low-side transistorwhen is conducting to generate a portion of the simulated inductorcurrent waveform when the high-side transistor is conducting.
 8. Themethod of claim 7 wherein the time when the inductor current isincreasing forms a base of a first triangle of the simulated inductorcurrent waveform; the time when the inductor current is decreasing formsthe base of a second triangle of the voltage across the low-sidetransistor when it is conducting forms a hypotenuse of the secondtriangle.
 9. The method of claim 8 wherein a hypotenuse of the firsttriangle which represents the simulated inductor current is increasingcan be determined from the values of the bases of the first and secondtriangles and the hypotenuse of the second triangle, whereby andaccurate complete simulated inductor current waveform can be generated.10. The method of claim 7 comprising generating a complete simulatedsawtooth inductor current waveform from the portion of the inductorcurrent waveform and the voltage across the low-side transistor when itis conducting; performing a cycle-by-cycle DC reset of the completesimulated waveform and the voltage across the low-side transistor whenit is conducting to avoid accumulating an error.
 11. The method of claim9 comprising performing a cycle-by-cycle DC reset of the completesimulated waveform and the voltage across the low-side transistor whenit is conducting to avoid accumulating an error.
 12. The method of claim7 wherein high-side and low-side transistors and control circuit are ona single integrated circuit (IC) and comprising: measuring a temperatureof the IC; storing values of Rdson of the low-side transistor as afunction of the IC temperature in a look-up table; utilizing the look-uptable and the measured temperature for adjusting the value of R Rdsonused to generate the simulated waveform the voltage across the low-sidetransistor when it is on.
 13. A power supply including a switchingconverter having a high-side switching transistor, a low-side switchingtransistor, an inductor and a control circuit for implementing currentcontrol of the power supply comprising: a circuit for monitoring avoltage at a switch node between the high-side and low-side transistorsto determine a time during which the inductor current is increasing anda time during the inductor current is decreasing wherein the voltageacross the low-side transistor when it is conducting represents a firstportion of a simulated sawtooth inductor current waveform; the controlcircuit utilizing the time when the inductor current is increasing, thetime when the inductor current is decreasing and the voltage across thelow-side transistor when it is conducting to generate a portion of thesimulated inductor current waveform when the high-side transistor isconducting, the control circuit implementing current control of thepower supply output.
 14. The power supply of claim 13 wherein the timewhen the inductor current is increasing forms a base of the firsttriangle of the simulated inductor current waveform, the time when theinductor current is decreasing forms the base of a second triangle ofthe simulated inductor current waveform and the voltage across thelow-side transistor when it is conducting forms the hypotenuse of thesecond triangle.
 15. The power supply of claim 14 wherein a hypotenuseof the first triangle which represents the simulated inductor currentwaveform when it is increasing can be determined from the values of thebases the first and second triangles and a hypotenuse of the secondtriangle whereby an accurate complete simulated inductor currentwaveform can be generated.
 16. The power supply of claim 13 comprisingmeans for generating a complete simulated sawtooth inductor currentwaveform from the portion of the inductor current waveform and thevoltage across the low-side transistor when it is conducting; means forperforming a cycle-by-cycle DC reset of the complete simulated waveformand the voltage across the low-side transistor when it is conducting toavoid accumulating an error.
 17. The power supply of claim 15comprising: means for performing a cycle-by-cycle DC reset of thecomplete simulated waveform and the voltage across the low-sidetransistor when it is conducting to avoid accumulating an error.
 18. Thepower supply of claim 13 wherein the high-side and low-side transistorsand control circuit are on a single integrated circuit (IC) andcomprising: a circuit measuring a temperature of the IC; a look-up tablestoring values for Rdson of the low-side transistor as a function of theIC temperature; a circuit utilizing the look-up table and the measuredtemperature for adjusting the value of Rdson of the low-side transistorused to generate the simulated waveform of the voltage across thelow-side transistor when it is on.